In recent years, advancement in semiconductor technology has been allowing a marked development of high-speed communication technology for mobile phones, personal computers, TVs and the like and also of high integration technology of communication equipments. Also, following advancement of information communication technology, in-circuit signals of electronic equipments and communication equipments in a low frequency band have been substituted by in-circuit signals in a high frequency band, and thus a circumstance has been changed to use electromagnetic waves in various frequency bands. On the other hand, integration of systems and diversification of used frequency bands have been making surrounding electromagnetic wave environments severe every day. For example, there has been a problem that, due to the system integration, many systems that have originally been independent from each other become in proximity to each other, causing interference with each other with electromagnetic waves transmitted therefrom and thereby inflicting dysfunction. In order to solve this problem, although electro-magnetic compatibility (EMC) during a product designing phase or a development phase has been required, it has been difficult to solve the problem of electromagnetic interference described above by means of the semiconductor technology alone.
Therefore, as one of measures against the problem of electromagnetic wave interference described above, a noise suppression sheet made of rubber or resin having magnetic material dispersed therein have been widely used. As the magnetic material for the noise suppression sheet, soft magnetic materials such as mainly metallic material and ferrite material have been used. As the metallic material, permalloy, amorphous and sendust are popularly used, and Ni—Zn ferrite and garnet ferrite are popularly used as the ferrite material. Heretofore, since the electromagnetic wave interference problem has been lying mostly in a range between a MHz band and a several GHz (3 GHz or so) band, the noise suppression sheets using existing magnetic materials (permalloy, sendust, Ni—Zn ferrite and the like) that exhibit high magnetic permeability in the above band were capable of dealing with the problem.
However, recent increase in use of a frequency band over 1 GHz, especially a frequency band over 5 GHz, associated with high-speeding of information communication technology has been leading to the advocacy of the electromagnetic wave interference problems in these bands. It is difficult to obtain a sufficient electromagnetic wave suppressing effect by using the noise suppression sheets using existing magnetic materials against noise at these frequencies.
For example, PLT 1 describes that, by controlling complex magnetic permeability and volume resistivity value of Ni—Zn ferrite, a noise suppression sheet that reflects less amount of radio waves of frequencies at 10 MHz to 1 GHz and has a large electromagnetic wave absorption may be provided. Indeed, Ni—Zn ferrite, because of its high complex magnetic permeability in the MHz band, is often used as a magnetic material having high noise suppression effect in the MHz band. However, the magnetic permeability sharply decreases at 1 GHz and becomes approximately 1 as a real part and 0 as an imaginary part at 5 GHz. For that reason, Ni—Zn ferrite may be considered as an unusable magnetic material from the viewpoint of noise suppression in the GHz band, for which there has been increasing demand in recent years. Further, most of other materials used as the magnetic material of the noise suppression sheet have high noise suppression effect between the MHz band and a several GHz (around 3 GHz) and, under the present circumstances, there is as yet no magnetic material that aims at the band over 5 GHz.
In view of the above, there is a need for a material that, unlike the conventional magnetic materials that exhibit high magnetic permeability in the MHz band, has high magnetic permeability in a GHz band. As a material that is expected to have such property, magnetoplumbite-type hexagonal ferrite that exhibits magnetic resonance in the GHz band may be mentioned. Magnetoplumbite-type hexagonal ferrite is a hard magnetic material and thus has been used as a permanent magnet, and there still are a number of researches to enhance its magnetic property. As an example of leading studies, there is element substitution. This is a method, by substituting some elements composing magnetoplumbite-type hexagonal ferrite with other elements, to improve initial magnetic permeability and coercive force, thereby enhancing performance as a magnet.
Regarding magnetoplumbite-type hexagonal ferrite, PLT 2 and PLT 3 have descriptions that magnetoplumbite-type hexagonal ferrite having various substituted elements may be used as a material having both high magnetic flux density and high coercive force and thus has improved performance as a magnet.